A control valve is generally used for a continuous control of a liquid or gas flow in different pipelines and processes. In a processing industry, such as pulp and paper, oil refining, petrochemical and chemical industries, different kinds of control valves installed in a plant's pipe system control material flows in the process. A material flow may contain any fluid material, such as fluids, liquors, liquids, gases and steam. The control valve is usually connected with an actuator, which moves the closing element of the valve to a desired position between fully open and fully closed positions. The actuator may be a pneumatic cylinder-piston device, for example. The actuator, for its part, is usually controlled by a valve positioner, also called as a valve controller, which controls the position of the closing element of the control valve and thus the material flow in the process according to a control signal from the positioner. The positioner is typically controlled with an electrical control signal from a control system (such as by a single twisted pair providing a 4 to 20 mA analog signal) and includes an electric-to-pressure (I/P) conversion to provide a pneumatic control for controlling the actuator.
One of the newer devices that offer improved performance of control valves is so-called “smart” positioner or a digital valve controller. A smart positioner is a microprocessor-based electronic positioner with internal logic capability which derives benefit from digital programming to obtain improved positioning performance. An advantage of the smart positioner is that it may be programmed to use a position control algorithm to achieve better dynamic response. Further, the smart positioner may use 2-way communications protocols such Hart, Foundation Fieldbus etc. to communicate with a process control system. This type of communication can be used also to enter new control settings or configurations remotely after installing a smart positioner.
However, sometimes there is a need to read measurements, make test runs, or change positioner settings locally at the positioner. Therefore, the smart positioners are often provided with a local user interface (LUI), or a control panel, enabling personnel to, for example, monitor the device behaviour as well as configuring and commissioning the positioning during installation and normal operation. A local user interface may be designed to have, for example, a display to present data and buttons, keypads, switches or other devices to operate the positioner and to enter parameters. For example, the local user interface may comprise a small LCD display and a key-pad with a small number of buttons. Often the display may be viewable through a window in a cover of the housing to allow showing some predetermined information without opening the housing. However, in many existing LUIs it is required that user must open the housing for any kind of operation of the LUI. An example of such approach is the electro-pneumatic positioner PositionMaster EDP300 from ABB Automation Products GmbH. Many oil and gas, petrochemical and process engineering plants are operating in harsh environments which require positioners to function in severe service applications, requiring reliable components with the ability to withstand extreme temperature or ambient fluctuations and have a chemical and corrosion resistance. The positioners must also have a sufficient shock resistance against external mechanical shocks.
The opening of the housing is typically cumbersome and time consuming as the cover of the housing is often closed by screws or similar means. The opening of the housing may sometimes be difficult (e.g. due to weather conditions or plant environment) or even impossible (e.g. forbidden by Ex regulations). Every opening of the housing will be a further risk for water ingress.
One approach could be to have buttons in the outer surface of the positioner so that they could be operated without opening the housing. For example, Universal positioner SRD960 from Foxboro Eckhardt GmbH has four external mechanical push buttons for local configuration and operation which penetrate the housing of the positioner. The mechanical switches may wear out or they may stuck due to the dirt. While external buttons might provide easy access, there is a new problem related to the security of the LUI usage. The ease of access may call for some protection to be implemented to prevent false input caused by dust, water drops, ice or other environmental sources. A simple keypad lock function may take care of this issue. Furthermore, when the user interface is accessible by not opening the housing, there is always a risk of un-authorized access on purpose or by human mistake. There is a need to prevent unauthorized use of local user interface especially when local user interface is available without opening the positioner cover and it is easy to access the device settings. One approach to solve this is to have PIN code protection to the devices to prevent unauthorized usage. However, it would be frustrating and time consuming to enter a PIN code (i.e. an access code) every time the local user interface is used, especially during start up configuration of the positioner. Further, there can be a high number of positioners and control valves, often hundreds of them, in a single plant, typically from several vendors, so that it would be a challenging task to manage and remember PIN codes for all positioners.